The semiconductor fabrication industry continues to source volatile metal containing precursors for vapor deposition processes, including chemical vapor deposition (CVD) and atomic layer deposition (ALD), for fabricating conformal metal containing films on substrates, such as silicon, metal nitride, metal oxide and other metal-containing layers, using these metal-containing precursors.
Barium and strontium containing precursors are especially sought after for the deposition of thin barium and strontium oxide containing thin films, such as: strontium titanate (STO) and barium strontium titanate oxide (BST) for advanced memory device manufacture. Strontium precursors are also useful for the thin film deposition of ferroelectric materials of the type SrBi2Ta2O9 for non-volatile memory, for the fabrication of thin film high temperature (Tc) superconductors of the type Bi2Sr2Can−1CunO5+(2n−1)d and for the manufacture of SrS:Ce and SrS:Cu phosphors for electroluminescent displays. Although there are fluorinated barium precursors which have excellent volatility, their use for BST manufacturing is effectively precluded, since fluoride ion can form in the oxide film and act as a charge carrier, which degrades the dielectric constant of the oxide film. Numerous strontium sources for strontium oxide and strontium titanate exist, but none have the ALD performance of the precursors of this disclosure.
Thus, there is a strong need for volatile unfluorinated barium and strontium precursor compounds with high ALD performance, but such compounds are scarce, especially so for barium. This stems from the large ionic radius of the barium +2 and strontium +2 ions requiring ionic ligands which can provide a coordinating environment sufficient to provide compounds which are monomeric or dimeric in strontium or barium. If this requirement is not met, the strontium, and especially the barium compounds tend to form highly associated or polymeric structures of limited volatility. However, even if monomeric or dimeric structures can be achieved, they may still not possess the thermal stability required to survive the high sublimation or distillation temperatures required for their vaporization and to maintain their thermal stability when adsorbed as monolayers in ALD. For all of these reasons, unfluorinated barium and strontium precursors, which are monomeric or dimeric, thermally stable, readily volatile and highly suited to BST and STO manufacture by ALD or CVD, are extremely scarce but highly sought after. Even more sought after are group 2 volatile precursors which have high deposition rates in ALD.
The prior art has attempted to provide precursors for these applications, as set forth below. However, none of the metal complexes in the prior art share the special characteristic of the complexes disclosed in the present invention. The compounds disclosed herein are exceptional in their volatility and thermal stability under conditions of vaporization. In addition, they have exceptionally high ALD deposition rates enabling them to grow 1 monolayer of metal oxide per ALD cycle. This makes them highly effective as precursors for STO and BST film growth and any other application which requires volatile sources of barium, strontium, magnesium, radium or calcium precursors.
Barium and other alkaline earth metal diketiminates are described in the literature. For beta-diketonates the compound [Ba(THD)2]4 (where THD is 3,3,5,5-tetramethylheptanedionate) has been reported. Although it is a stable and volatile compound, its molecular weight of >2000 renders its utility in ALD or CVD process challenging, especially when compared to smaller, more volatile, compounds, such as the barocenes and especially when compared to the new compounds of the present invention, which are more cleanly volatile and have higher ALD deposition rates and lowered melting points when compared to the barocenes.
Thermally stable and volatile tri(pyrazoyl)borate alkaline earth complexes, including those of barium have been reported. However, these compounds suffer from containing the element boron, which under oxidation conditions, deposits boron oxide into the metal oxide of a CVD or ALD process.
Volatile barium and strontium complexes based on diketiminate ligands bearing NMe2 groups substituted on the core diimine nitrogens are reported. But the barium molecule in this series sublimed to give only a 79% sublimation recovery leaving a 14 wt % involatile residue. This sublimation was performed at 0.05 Torr. If it had been conducted at atmospheric pressure, its involatile residue would have been significantly higher. These results indicate this molecule to have limited suitability as a precursor for CVD or ALD processes.
The synthesis and thermal properties of a series of nine different barium cyclopentadienyl, also known as barocenes', have been reported. Selected species from these barocenes, particularly the barium bis(tri-tert-butlycyclopentadienyl), are ‘state of the art’ precursors possessing an attractive combination of thermal stability and volatility. However, the new molecules of the present invention are shown to be superior to them with regard to these two key properties. Additionally, the new precursors of this disclosure, due to their unique molecular characteristics, also have exceptionally high ALD deposition rates making them more attractive from a device manufacturing perspective.